Rare earth metal-based permanent magnets such as R—Fe—B based permanent magnets (where R represents a rare earth element inclusive of Y) are used nowadays in various fields because of their high magnetic characteristics, and the demand is recently increasing.
However, since R—Fe—B based permanent magnets contain a highly reactive rare earth element: R, they are apt to be oxidized and corroded in ambient, and in the case they are used without applying any surface treatment, corrosion tends to proceed from the surface in the presence of small acidic or alkaline substance or water to generate rust, and this brings about the degradation and the fluctuation of magnetic characteristics. Moreover, in the case such a rusty magnet is embedded in a magnetic circuit and a like device, there is fear of scattering rust to contaminate peripheral components. In the light of the above circumstances, referring to Patent Literature 1 and the like, there have been disclosed rust preventive treatments of forming a Ni plating film, a Cu plating film, or a combination of both, as surface treatments of rare earth metal-based permanent magnets, and those methods have been widely employed in the art.
On the other hand, in the case of embedding a joined structure obtained by joining a rare earth metal-based permanent magnet having a Ni plating film at the outermost surface thereof with other member using an adhesive in various devices, it is required that the Ni plating film and the other member exhibit strong adhesion properties via the adhesive. However, due to the effect of the passivation film that is formed on the surface of the Ni plating film, there are cases in which the adhesion properties become inferior to that of a resin film or an aluminum film depending on the usages, which causes problematic adhesion failure.
In order to overcome this problem, Patent Literature 2 proposes a technique comprising pickling the surface of a Ni plating film with an organic carboxylic acid. This is an excellent technique for recovering the adhesion properties of a Ni plating film.
However, when the joined structure adhered by the method proposed in Patent Literature 2 is allowed to stand under a moisture resistance test, a decrease in the adhesion strength has been observed. In particular, a distinct decrease is observed in the case a silicone-based adhesive is used as the adhesive.
Patent Literature 3 and Patent Literature 4 disclose techniques comprising applying a single layer Cu plating or a single layer Ni plating on the surface of a magnet containing a rare earth element, followed by carrying out a Cu alloy plating, thereby obtaining a rare earth magnet having high magnetic characteristics and excellent corrosion resistance.
However, these patent literatures contain no considerations on the improvement of the adhesion properties, and have no disclosures on the constitution of the base plating film for improving the adhesion properties or on the composition of the Cu alloy film.    Patent Literature 1: JP-A-1-321610    Patent Literature 2: JP-A-2003-193273    Patent Literature 3: JP-A-2007-273503    Patent Literature 4: JP-A-2007-273556